The present invention is in the field of glass-ceramic articles and particularly relates to glass-ceramic articles having different and adjacent crystal phase areas. Stated another way, this invention relates to the field of variably translucent glass-ceramic articles suitable for use as stove tops and fire-door windows.
Recent developments in range-top design such as the radiant halogen burner require the transmission of near infrared and red visible light to allow both rapid heating and a visual indication of heat transfer. Up to now, materials which accomplish these ends have been dark transparent stuffed .beta.-quartz glass-ceramics doped with vanadium or cobalt to give a black appearance in reflected light, but transmit in the red and near infrared. These materials have low or near-zero thermal expansion characteristics, but are not typically strong mechanically, with abraded flexural strengths well below 10 ksi or 70 Mpa.
U.S. Pat. No. 4,211,820 (Cantaloupe et al.) discloses such a glass-ceramic material. This transparent glass-ceramic, useful for smooth-top cooking stoves, displays a warm brown coloration sufficient to conceal a heating element when not in use but permitting visible observation thereof when in operation. Compositionally, this material consists essentially, expressed in terms of weight percent on the oxide basis, of about 2.5-4.5% Li.sub.2 O, 1-2% MgO, 19.5%-21% Al.sub.2 O.sub.3, 66.5%-68% SiO.sub.2, 4-5% TiO.sub.2 and 0.02%-0.2% V.sub.2 O.sub.5. This glass-ceramic article consists of a surface layer composed of .beta.-quartz solid solution crystals as the predominate phase with, optionally, a minor proportion of .beta.-spodumene solid solution dispersed in glassy matrix, while the interior portion contains .beta.-spodumene solid solution crystals as the predominant crystal phase dispersed within a glassy matrix.
A variation of the above disclosed glass-ceramic is described in U.S. Pat. No. 4,835,121 (Shibuya). This patent discloses a strengthened, infrared transparent glass-ceramic article consisting of .beta.-quartz solid solution crystals alone dispersed within a glass matrix and displaying, a thickness of 3 mm, a dark or black appearance, a transmittance of about 5% or less for a visible of a wavelength of 500 nm or less, and a transmittance of about 80% or more for infrared radiation at a wavelength of 1,500 nm, as well as having a bending strength of 20,000 psi or more. Compositionally, this glass-ceramic consists essentially, by weight of 60-72% SiO.sub.2, 14-28% Al.sub.2 O.sub.3, 2.5-5.5% Li.sub.2 O, 0.1-0.9% MgO, 0.1-0.9% ZnO, 3-6% TiO.sub.2, 0.03-0.5% V.sub.2 O.sub.5, 0.1-1% Na.sub.2 O, 0-1% K.sub.2 O, 0-2% CaO, 0-2% BaO, 0-2% PbO, 0-2% As.sub.2 O.sub. 5, 0-3% ZrO.sub.2, and 0-3% P.sub.2 O.sub.5.
In either of these above described glass-ceramics, the phase transformation from metastable .beta.-quartz solid solution to stable .beta.-spodumene solid solution would normally involve a dimensional change of usually over 1% Therefore, in utilizing these compositions for large articles like a range-top, it is likely that this phase transformation change would result in cracking due to dimensional variations across the article. For this reason, glass-ceramic articles, like those above, usually contain either all .beta.-quartz or all .beta.-spodumene crystals so to avoid this excessive dimensional change. However, some glass-ceramic articles do contain mixtures, i.e., a dominant crystal phase with trace amounts of a minor phase, but these articles have not undergone a complete phase change which would result in large dimension changes. These mixed, dominant/minor phase systems, however, usually result in glass-ceramic articles with diminished mechanical properties.
Therefore, the primary objectives of this invention are:
1. to provide for a glass-ceramic which may contain distinct areas of different crystallization adjacent to each other through different crystallization; specifically, .beta.-quartz areas adjacent to .beta.-spodumene/gahnite mixed phase areas, which results in a glass-ceramic with variable translucency across its dimension;
2. to provide for a glass-ceramic which undergoes near zero dimensional change as it thermally changes from the .beta.-quartz crystal form to the .beta.-spodumene crystal form; and,
3. to provide for a transparent, clear and essentially colorless glass-ceramic article which has the ability to change rapidly to the opaque state without the large dimensional changes which typically result in cracking of the article.
Utilizing this type of material for flat stovetops would be quite ideal, both safety-wise and aesthetically. The resultant stovetop could be comprised of a variably translucent sheet--clear or slightly translucent burner areas in an otherwise opaque white colored sheet. Alternatively, the overall stovetop could also be of one translucence.
Additionally, this glass-ceramic would be ideal for fire door windows. Typically, prior art glass-ceramics, when exposed to high temperatures, such as that of flames, would crack due to excessive internal stresses created when specific areas were phase changed, while other, adjacent areas were not. In other words, the dimensional change experienced in the phase changed areas of the glass-ceramic, as compared to the other non-transformed areas, was large enough to cause cracking.